The trend in the integrated circuit (IC) industry is to develop circuit boards with reduced weight, reduced thickness, and reduced dimension. However, because of the demand in increased performance, the trend in IC chips is to increase its size. The need to package IC chips with ever-increasing dimensions in a circuit board with ever-decreasing dimensions, while maintaining excellent reliability and dielectric properties, presents a great challenge to the IC packaging industry.
Utilization of the lead-on-chip (LOC) packaging technology can reduce the area of a plastic-based circuit board. Typically, the LOC technology allows the chip surface to achieve 85% of the total board surface, an improvement of about 30% over conventional technologies. Thermosetting epoxy resins are commonly used as the adhesive tape in LOC applications. However, because of their relatively high ionic contents and high volatile material, the reliability of epoxy based LOC packages, especially with regard to dielectric properties, often is not very reliable. This problem can become even more profound when the IC products are moving toward high production rate and high clock speed.
Polyimide (PI), which is a polymeric material with high heat resistance and good dielectric properties, has been widely used in electronic applications, such as in making flexible printed circuit (PFC), tape automated bondings (TAB), etc. More recently, PI is also gradually used in IC packaging applications as an adhesive. Prior art polyimides typically are either non-thermoplastic resins which do not exhibit flow behavior at elevated temperature (i.e., above Tg), or are thermoplastic resins but with a very high Tg which would degrade before it exhibits any flow behavior. Furthermore, in order to improve the adhesion of polyimide resins, various epoxy resins are often added to form a polyimide/epoxy resin mixture. However, this can adversely affect the heat resistance of the resultant mixture.
Both non-flowable thermoplastic and thermosetting polyimides are not suitable for use as the adhesive tape in LOC applications. In order for polyimides to be used as adhesive tapes in LOC applications, they must be thermoplastic in nature and exhibits a definitive micro-flow behavior, so as to facilitate the adhesion between the lead frame and the IC device. However, the amount of adhesive flow of the polyimides cannot be too great. An excessive extent of the adhesive overflow due to high flowability will affect the later stage packaging process and, more importantly, it will cause a stress to be generated which can affect the reliability of the final packaged product. In other words, a desirable polyimide, in addition to its being thermoplastic in nature, must have enough molecular movements at temperatures above Tg, so as to achieve good contacts with the lead frame. However, too much of such resin mobility can result in adhesive overflow, this is undesirable. At the present time, no polyimides can simultaneously satisfy both of these requirements.
U.S. Pat. No. 5,200,474, the content thereof is incorporated herein by reference, discloses a polyimide adhesive composition which is a reaction product of polyamic acid and bismaleimide modified by barbituric acid or a derivative thereof. The polyimide adhesive exhibits excellent adhesion with copper film, but it has a very high Tg, and does not provide enough fluidity for LOC adhesive tape applications.
U.S. Pat. No. 5,268,432, the content thereof is also incorporated herein by reference, discloses a heat resistant adhesive composition comprising an admixture of a modified bismaleimide (BMI) resin, a modified polyamide-imide and a solvent. The bismaleimide is modified by barbituric acid and/or a derivative thereof and the polyamide-imide is modified by an epoxy resin. The overall solid content of the heat resistant adhesive composition is in the range of about 15 to 50 percent by weight, wherein solid contents per overall solid content of the modified bismaleimide resin and the modified polyamide-imide are 60 to 90 percent and 10 to 40 percent, respectively. Similar to polyimide adhesive composition of the '474 patent, the polyimide based adhesive exhibits excellent adhesion with copper film (and with improved heat resistance), but it also has a very high Tg, and does not provide enough fluidity for LOC adhesive tape applications.